The app is set up in the tables of Shipping Agents, Shipping Agent Services and Shipment Methods.
You can either use the necessary package lines for the GLS packages in inventory picks, for this you have to switch on the picking in the storage locations:
Location
Require Pick
To use a storage location for picking, the checkbox "Require Pick" must be set.
Or you can also use the simple variant directly in sales orders and purchase return orders.
To do this, you have to activate in the Shipping Agents setup that the package lines should be displayed.